1. Field of the Invention
The present invention relates to a rear door structure for a vehicle, and particularly to a flip-up type rear door structure hinged to an upper rim of a rear end opening of the vehicle.
2. Description of the Related Art
Conventionally, as a flip-up type rear door structure, there has been known a structure having an inner panel including: a frame part which corresponds to a contour of a rear door; and a lateral frame portion integrally formed with the frame part that separates the frame part into an upper portion and a lower portion (see, for example, Japanese unexamined patent publication No. 8-258568 (paragraphs 0010-0015 and FIG. 1)). FIG. 25 shows a schematic diagram illustrating a conventional rear door structure seen from a rear side of a vehicle. As shown in FIG. 25, a rear door structure 101 of a vehicle V has an inner panel 106 including a frame part 109 and a lateral frame portion (or also called “lateral beam”) 120. On an upper side of the lateral frame portion 120, an opening 112a is bounded by the frame part 109 and the lateral frame portion 120, into which a rear window 107 is installed. On a lower side of the lateral frame portion 120, an opening 112b is bounded by the frame part 109 and the lateral frame portion 120, in which two separate reinforcement parts (connecting reinforcements) 110a, 110b are arranged in an approximate V-shape. A lower end of each of the reinforcement parts 110a, 110b is welded to a lock reinforcement 111 joined with a center portion of a lower bar portion of the frame part 109, and the reinforcement parts 110a, 110b extend to their respective upper corners of the opening 112b. Upper ends of the reinforcement parts 110a, 110b are welded to corner reinforcements 130a, 130b, respectively, which are joined with the upper corners of the opening 112b where the frame part 109 and the lateral frame portion 120 meet.
In this rear door structure 101, rigidity of the inner panel 106 is enhanced by two separate reinforcement part 110a, 110b arranged at the opening 112b. 
However in the conventional rear door structure, though rigidity of the rear door is enhanced, introduction of the reinforcements creates a problem of weight increase, and welding of the reinforcements creates a problem of an increased number of production steps. In addition, when rigidity is not sufficiently high, a space bounded by the lateral frame portion 120 and the reinforcement parts 110a, 110b may vibrate when the vehicle V is running or an engine thereof is idled. This vibration is transmitted to a vehicle interior as unpleasant noise and vibration (NV), and may disturb quietness of the vehicle V.
Therefore, it would be desirable to provide a rear door structure for a vehicle in which rigidity is sufficiently secured while the number of production steps does not increase, as compared with the conventional rear door structure. It would be further desirable to provide a rear door structure for a vehicle in which quietness of the vehicle is secured by enhancing rigidity.